1. Field of Invention
This invention relates generally to an electrophotographic copying apparatus, and more particularly, to the heat and pressure fixing of toner images formed on a copy substrate by direct contact with a heated fusing member.
2. Description of Related Art
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent development of the latent image by the application of marking particles commonly referred to as toner. The visual toner image is typically transferred from the member to a copy substrate, such as a sheet of plain paper, with subsequent affixing of the image by one of several fusing techniques.
In one type of fusing system, a fuser roll is used which has an outer surface or covering of polytetrafluoroethylene or silicone rubber, the former being known by the trade name “Teflon”, to which a release agent such as silicone oil is applied. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
Another type of fusing system includes a primary fuser for fixing toner images formed on either side of a support material. The support material is run through the primary fuser for fixing of toner on a first side and then run through the primary fuser a second time for fixing of toner on a second side of the support material.
Known fusing devices handle many types of print mediums, such as, for example, paper, card stock, poster board, or the like. Heavier weight and many coated copy mediums require more energy to be fused. Lighter weight copy mediums require less energy for fusing. Too much energy applied to a light weight copy medium, such as, for example, paper, results in over-fusing of the paper and non-uniform cooling. Over-fusing of the paper and/or non-uniform cooling results in paper deformation, paper wrinkle, and other print defects. The deformations in the paper will set once the paper cools. Further, too much energy may also result in difficulties in stripping the paper off the roll because the paper will become “flimsy” when overheated.
Application of more energy than required for a given print medium will also tend to negatively affect the quality of the toner. More specifically, the toner may split (leaving some toner on the fuser roll and some on the image on the medium) and/or offsetting may occur (re-depositing some or all of the toner to the next print). This may further lead to contamination of the fusing system.
Not enough energy applied to a heavier weight copy medium, such as, for example card stock, will result in the toner image not being fully fused, causing print defects. Simply setting the fuser roll 16 and pressure roll 18 at a lower temperature to accommodate light copy mediums, and then, for example, passing heavier weight copy mediums through the primary fuser system 10 more than once will not yield satisfactory results. With multiple pass fusing, by the time the copy medium gets transported back to the pre-fuser transport 12, the paper is already cooled to a point where there would be no added benefit in passing the paper through the fuser again.
The primary fusing device 10 is designed to handle a full range of paper weights. However, heavier stocks, such as, for example 280 gsm uncoated or coated stocks, require a substantial amount of energy to be put into the paper to fuse, for example, four layers of toner (cyan, black, magenta and yellow) at a level that is acceptable to, for example, the graphic arts industry.
Further, the primary fusing device 10 must handle lighter stocks, such as, for example, 50 gsm uncoated and coated stocks, without over fusing these lighter weight stocks. Heavy weight papers, especially smooth coated heavy weights, are proving to be difficult to fuse because so much energy is being put into the paper that gloss differential on the first side image in the paper of the pre-fuser transport occurs due to non-uniform cooling effects of the post fuser vacuum transport and pinch roller transports. Further, increasing fuser nips and raising temperatures with thicker-coated rolls will not allow for medium to low beam strength copy mediums (i.e., light weight to medium weight mediums) to strip with primary fusing device 10.
Fusers must also handle high gloss papers. With high gloss papers, too much energy applied to the paper will result in image artifacts, such as, for example, a false image of the post-fuser transport and pinch roller due to non-uniform cooling. Further, to fuse a heavier weight substrate with merely a single fuser system, visible defects will result. Further still, the light weight substrates will be compromised.
There is a need for a fuser that can adequately fuse a full range of copy medium substrates. A single fuser operating space to achieve the performance goals on all substrates has not been possible. The present invention provides a full system solution wherein a secondary fusing system, in series with the primary fusing device 10, is provided to accommodate a range of print substrates.